1. Field of the Invention
The invention relates in general to the protection of electrical reactors, and more specifically to the detection of shorted turns in an electrical reactor.
2. Description of the Prior Art
An EHV transmission line produces certain VAR requirements upon the two end systems. These requirements increase as the square of the voltage, and they are a function of the line capacitance and length. If the systems are unable to absorb the VARs with inductive loads or underexcited machines, terminal voltages may rise excessively under light load conditions. A common arrangement for absorbing these VARs is by the use of shunt reactors located near the ends of the EHV line sections. Shunt compensation may be in the form of electrical reactors connected directly to the line being compensated, to the tertiary windings of the step-up or step-down transformers at the terminals of the line, or on the lower voltage bus to which the transformers are connected.
Shunt reactors are conventionally protected in the prior art by a variety of complementary and/or overlapping protective relay functions, including overcurrent, differential, and impedance relays. Overcurrent relays respond to all types of faults, but they have limited sensitivity. Differential relays are very sensitive to winding-to-ground faults, but they do not respond to turn-to-turn faults. Impedance relays, looking into the reactor from the line side, can detect a drop in impedance caused by a turn-to-turn fault, as long as enough turns are shorted. In other words, the impedance relay must be set such that it will not trip the associated circuit breaker, or breakers, for the minimum normal impedance of the reactor. The normal impedance of a reactor varies significantly with heating and voltage. Thus, a turn-to-turn fault which does not drop the reactor impedance below the lower end of the normal impedance range will go undetected. It would thus be desirable to be able to supplement these conventional forms of reactor protection with a protective function which is sensitive to turn-to-turn faults, even when the shorted turns do not drop the impedance below the lower end of the normal impedance range of the protected reactor.